Astronomy Now April 1996
1996 marks the 30th anniversary of the first successful soft-landing Moon-probes. The controlled setting-down of the Soviet spacecraft Luna 9 and Luna 13, and the beginning of the US Surveyor programme, represent a series of hard-won interplanetary "kisses" after centuries of distant admiration.
The Luna Probes
Initial attempts at controlled lunar landings met with disaster. In 1965 the Soviet Union dispatched four Luna soft-landers. Luna 5 impacted on the lunar surface in Mare Nubium at an unhealthy velocity after a retro-rocket failure. A large cloud of dust thrown up at the impact site was reputedly observed by the German observatory at Rodeswich. The spacecraft (and all other probes which have hit the Moon at breakneck speed) gouged-out a substantial impact crater tens of metres in diameter. The next probe, Luna 6, missed the Moon by 150,000 km and is still in an independent solar orbit. Luna 7’s retro-rocket fired too early, and the spacecraft, travelling at 3,500 km/h, dived into the Oceanus Procellarum just west of Kepler. Luna 8 applied its rocket brakes a fraction too late, and at a speed of 70 km/h the probe churned-up the lunar soil northwest of the unusual formation Reiner Gamma (a young lunar feature which may be the result of the impact of a cometary nucleus).
The first (unsuccessful) impact site of Luna 5, near landsberg. Crater drawing by Peter Grego made using a 60 mm refractor.
Soft-landing success finally came in February 1966 with Luna 9, which landed on the flanks of a 25 metre craterlet around 40 km northeast of Cavalerius in the Oceanus Procellarum. The landing area was named "Planitia Descensus" (Plain of Descent) to commemorate the event. Photographs of the surface transmitted by Luna 9 were first revealed to the world not by the Soviet news agency TASS, but by astronomer Sir Bernard Lovell at the Jodrell Bank radio telescope. In what was a remarkable scoop, Jodrell Bank had picked up the craft’s transmissions and each of the pictures had been assembled with an ordinary newspaper telephoto machine. The probe showed that the Moon’s surface - at least, in the neighbourhood of the craft - was not covered with a thick layer of dust capable of completely engulfing later landing missions. The pictures, showing small features about 2 metres away to a horizon several kilometres distant, provoked a barrage of speculation as to the nature of the lunar surface. One British "expert" confidently announced, on the basis of a just one brief glance at an indistinct newspaper picture:-"It now seems very likely that much of the surface material is volcanic in origin, and that the craters are a result of internal mechanisms, not the impact of meteorites."
Lunas 10, 11 and 12 were orbiting spacecraft whose sensors obtained valuable scientific data in near-Moon space. The lunar magnetic field was found to be weak, and no radiation belts were discovered.Luna 13 soft-landed in the Oceanus Procellarum, 40 km southeast of the crater Seleucus, in December 1966. The mission called for a more intense scrutiny of the surface than its predecessor, Luna 9. Though similar in appearance to Luna 9, Luna 13 was a much more complicated piece of machinery. Not only was it capable of taking high resolution television pictures, it also possessed extending arms which sampled the lunar soil (termed "regolith") for various properties. The camera could complete a full panoramic sweep in around an hour and a half, and it had a focus range of 1.5 metres to infinity. This meant that particles as small as 1.5mm could be resolved in the foreground. The terrain in the craft’s vicinity was found to be rather flat and featureless, with only small craterlets visible in the distance. The density of the regolith was tested with an explosive hammer, struck with a force of 7 kg onto the surface. It was found that the regolith, to a depth up to 30 cm, was similar in density to terrestrial soil.
Surveyor on the Moon
The Surveyor programme, from 1966 to 1968, landed sophisticated probes at various locations on the Moon’s near-side. Among Surveyor’s objectives were to return TV images and provide engineering performance data; the later probes conducted intricate chemical analyses of the lunar regolith. Their descent to the surface, controlled by radar altimeter, was facilitated by a large central retro-rocket (discarded at a height of over 11 km) and three small vernier rocket engines which cut-out just 4 metres above the Moon’s surface, allowing the spacecraft to gently land at walking speed. Five out of the seven Surveyors were successful.
Surveyor 1 soft-landed on the flat plains of Oceanus Procellarum, some 40 km north of Flamsteed, in June 1966. 11,237 images of the surrounding landscape were taken (some of them in colour), with a resolution up to 0.5 mm at a focus of 1.2 metres. Perhaps the weirdest image to have been returned from the Moon’s surface was a 45 cm boulder which resembled the fossilised head of a dinosaur - complete with eye socket, nose and mouth. Alas, it turned out that this "Lunasaurus Improbabilis" was nothing more than an unusually shaped lump of basalt. Three months later Surveyor 2 worked perfectly prior to landing, until a malfunction caused it to rotate wildly and uncontrollably, eventually forcing a crash 300 km south of the crater Eratosthenes.
Surveyor 3 soft-landed in Mare Insularum in April 1967, 40 km southeast of the site of the crashed Luna 5. The probe returned 6,315 pictures of the environment, and it was discovered that the probe rested on the gentle slopes of a small crater some 200 metres in diameter - this crater, which can’t be seen in small telescopes, was named Surveyor crater to honour the occasion.With its remotely controlled regolith scoop, Surveyor 3 dug an 18 cm deep trench in the lunar topsoil whilst the cameras observed from above. A watch of some 18 hours was kept on the disturbed site to monitor and record any physical change. The actions of the scoop suggested the upper regolith had the mechanical properties of fine, damp sand, and that it would be supremely capable of supporting the weight of manned activities. 31 months later, in what was a remarkable a feat of pin-point landing accuracy, Apollo 12’s Intrepid touched down a mere 200 metres northwest of Surveyor 3. Parts of the Surveyor were removed by the astronauts to see how the probe had fared in the harsh lunar environment.
In July 1967 Surveyor 4 was to fail just minutes from the planned landing; it was presumed to have crashed in the Sinus Medii just north of the crater Oppolzer. Two months later, and after several potentially disastrous in-flight malfunctions, Surveyor 5 made a successful landing 70 km south of the Lamont formation in Mare Tranquillitatis. As well as taking more than 18,000 TV pictures, the probe conducted an analysis of the regolith using an alpha-ray scatterer, a sensitive instrument which was capable of identifying surface material by firing alpha-rays at the lunar soil and then measuring the properties of the reflected energy (which varies with each element). The observations indicated that the regolith was a silicate material of basaltic composition, a conclusion supported by the soil’s iron content, detected by small magnets attached to the probe. Less than 2 years later the environment was examined in greater detail when Apollo 11’s Eagle touched down just 30 km south of Surveyor 5.
Following hard on the heels of its predecessor, Surveyor 6 landed a few kilometres north of the crashed Surveyor 4 in the Sinus Medii. An enormous number of TV images were obtained in November 1967 - no less than 30,065 of them. The chemistry of the regolith was also placed under intense scrutiny. Ten days after landing the probe’s vernier rocket engines were deliberately re-fired for 2.5 seconds in order to examine the impression the probe’s landing pads had made in the Moon’s soil. Surveyor 6 made a small hop and landed 2.5 metres from where it had originally set-down.
The last probe in the series landed in a mountainous region only 20 km north of the rim of the prominent crater Tycho. During January 1968 Surveyor 7 returned 21,274 TV pictures and sampled the regolith. Of the most interesting photographs were those showing two Moon-ranging laser beams emanating from observatories on the Earth’s night side.
Encouraged by lunar soft-landing success, spacecraft designers knew that the surfaces of the terrestrial planets would soon be explored in exquisite detail. Within a decade of the first lunar soft-landing the remains of robot and human activity were strewn about the surface of Moon. The planet Venus had grudgingly allowed several Soviet Venera spacecraft to report on atmospheric and surface conditions before their swift destruction, and spectacular images of the Martian desert were returned by the two US Viking landers.
